Many vehicles include computers that are programmed to perform on-board diagnostics to identify systems that may not be operating within specified limits. These diagnostic tests are typically performed in real world conditions, for example, while the vehicle is operating, or just following operation of the vehicle. Accordingly, the results of the tests may depend on uncontrolled environmental factors and operating conditions of the vehicle such as the geographic location of the vehicle, weather conditions at the time of driving and/or at the time of the test, usage patterns of the vehicle, etc. However, present test specifications do not account for various uncontrolled (or uncontrollable) factors and may therefor provide unreliable, inaccurate, or at least less accurate than is desired, diagnostic test results.